demo.scala

import com.typesafe.config.Config
import classy.core.Decoder
import classy.generic.derive._
import classy.config._

case class Bar(value: String)
case class Foo(
  a: String,
  b: Option[Int],
  c: List[String],
  bars: List[Bar]
)

val decoder: Decoder[String, Foo] =
  MkDecoder[Config, Foo]
    .renameFields(_.toUpperCase)
    .decoder.fromString
  
val res = decoder.decode("""
   A = OKAY
   B = 1235
   C = [ZZZ]
   BARS = [{ VALUE: hi }]
 """)
// res: Right(Foo(OKAY,Some(1235),List(ZZZ),List(Bar(hi))))